Before iOS 8, you could only 'share' your current location via the Maps app.
Now you can do it in iMessage.

"Send Current Location" in iMessage, means just that - you send your current location this one instance in one message. Because, hmmm..., you want the ambulance driver to find you before you'zdead.

"Share your current location" means you can keep sharing your location for the duration of a message threat, over an entire conversation with everyone involved. You can specify for how long you want to continue sharing etc... The setup is popular with parents who want to always know were their chatty kids are in time and space.
Enable either, and certain options will show up.